Wow, that's a tougie. You lean like in paper mario, i'm assuming?
Well people have made 2D games with 3D components so im sure you can have a 3D game with 2D components.
But i know you can't use sprites for it. You'd have to make the 2D object as a 3D object...you could just draw it and give it no Z depth, or something. That kind of idea.
alternatively you could make the sprite on the screen so that it's on top of everything. However that means you couldn't rotate a camera around it.
Sorry if i'm not much help, i'm still learning-but i do a lot of experimentation.
Your soul...it tastes like chicken
